1. Field of the Invention
The present invention relates to an illuminating optical device which may be used in optical apparatuses such as photomechanical process cameras, projection testers, steppers and film and other projectors. More particularly, the present invention pertains to an illuminating optical device that employs a fly-eye lens.
2. Description of Related Art
The fly-eye lens array is generally known as an optical element that converts information about the angle of light rays incident thereon into positional information. Namely, the rays of light which are incident at several different angles to a fly-eye lens element can be focused on their own focussing points individually, and the rays of lights focused thereon will be diffused from the focused points respectively. Consequently, it is employed as a light diffusing element for diffusing the light from a light source to illuminate an object uniformly. For example, when, in photomechanical processing system, a block copy film having the image of an original formed thereon by photographic process is illuminated to project the image of the film onto a presensitized plate through a projection optical system, the fly-eye lens array is disposed between the light source and a condenser lens to uniformly diffuse the light from the light source and illuminate the block copy film disposed between the condenser lens and the projection optical system with the uniformly diffused light. The fly-eye lens array has higher light diffusing effectiveness than that of a diffusing plate and it is therefore capable of illuminating an object even more uniformly.
Fly-eye lenses are generally employed in a part where light is condensed with a view to minimizing the overall size of the system and, therefore, a glass material having superior heat resistance, for example, quart glass, is employed to form fly-eye lenses. Quartz glass is costly and the cost of machining quart glass into a minute lens constituting a fly-eye lens is exceedingly high. In particular, the polished surfaces at the incidence and emergence sides of a fly-eye lens are required to have an extremely high degree of surface finish, and if the finished surfaces have flaws or sand marks left thereon, the lens is rejected as a defective. For these reasons, the prior art suffers from high production cost.